Quite a bit of what has been said so far is not correct. The 3c stamp pays the standard letter fee and the 1c stamp pays the "to the mails" fee. Fees for delivery were never prepaid nor were they paid for with stamps, they were collected in cash. So, this cover was picked up by a mail carrier at the cost of 1c. The 3c paid for the letter to be taken to Wolcottville, CT.
doug2222, You need to pay more attention, I sold two Civil War Patriotic carrier covers on
ebay last year. Carrier covers are not that common because not many cities had a carrier department.